While the present invention has broad application in the field of aqueous polymeric coating compositions and their preparation and use, the impetus for the invention's development arose from a desire to provide superior vehicles for printing inks. Due to the recognition in recent years of serious environmental problems associated with organic solvent-based inks, attention has been directed to the development of organic solventless inks, including aqueous inks.
As the result of such research, two general types of solventless ink systems have been introduced commercially--i.e., catalytic and UV curing inks. The catalytic inks comprise liquid prepolymers and a stabilizer. After printing, the printed material is heated to a temperature at which the stabilizer decomposes, allowing the prepolymers to crosslink, forming a film of dry, thermosetting resin. The UV curing inks comprise liquid photopolymers and initiators which release free radicals upon irradiation by UV light. The free radicals initiate polymerization of the photopolymer to a solid film of dry thermosetting resin.
The foregoing solventless ink systems have certain limitations in terms of quality of print, speed of setting and smoothness and adherence of the resulting film. Particular shortcomings of such inks and ink systems, in general, have been the extreme toxicity of certain materials found in the UV systems and lack of stability of the catalyzed inks. Attempts to use water as the ink solvent have resulted in lack of adhesion to certain polar substrates, such as polyolefins--e.g., corona discharge-treated polyethylene which is widely used, e.g., in packaging and which has proved to be quite difficult to print because of poor adhesion of inks. One approach to a water-based ink is disclosed in commonly-assigned, copending application Ser. No. 687,413, filed May 17, 1976, now U.S. Pat. No. 4,072,644, by Hedrick, disclosing aqueous-based printing inks utilizing a polymer solubilized in the ink by a rapid removable solubilizing component, the polymer being capable of forming a water-insensitive gel film upon removal of the rapid removable solubilizing component.
Therefore, I set about to design improved organic solventless, aqueous polymer latex coating compositions and, particularly, such compositions which would be useful as vehicles for ink compositions capable of producing quick-setting, smooth and adherent films when printed on a variety of substrates, including difficult-to-print substrates, such as corona discharge-treated polyethylene or the like. Although my invention as hereinafter described enables one to produce smooth, adherent coatings from aqueous latex systems, it will be apparent that the underlying principles are also applicable to latex-type formulations based on organic solvent systems, such as organosols, as long as the discrete polymer particles can be suspended therein and the second discrete component can be added in a compatible manner.